This invention relates to sound pickups for tracking mechanically modulated grooves in a record support by means of a tracing stylus, of the type in which a stylus holder is elastically mounted, at its center of motion, in an elastic bearing element such as a bearing element of rubber and rubber-like material. More particularly, the present invention is directed to an improved pickup mounting of this type.
In modern pickups of this kind, the grooves are tracked by means of a diamond which is ground to a spherical, elliptic or other spiral shape. Such a diamond is provided at one end of a stylus holder. Ordinarily, the stylus holder is mounted in the pickup in elastic material, such as rubber, having quite specific properties, either near the theoretical center of motion or at the theoretical center of motion itself.
The position of the theoretical center of motion is determined by the geometry of the arrangement. If, for example, the stylus holder is provided in the form of a homogeneous rod or tube of the length L, the optimum position of the theoretical center of motion will be found at a distance of L/6 from the center of mass of the stylus, and in that portion of the stylus holder extending in the interior of the pickup housing. For the theoretical center of motion determined in this manner, the mass to be moved relative to the stylus tip, which is represented by the diamond, is minimized. Therefore, it is important to locate the bearing element for the stylus holder which is made, preferably, of rubber, as close as possible to the theoretical center of motion, particularly if the bearing element is relatively large, because then, due to the just-mentioned measure, the necessarily co-vibrating masses cannot have a too great reactive effect on the stylus tip.
In known pickups, in order to define the spring element more exactly, a thin, elastic wire is frequently provided at the center of motion, and this wire, on the one hand, is engaged in the bearing element and, on the other hand, is connected to a fixed point of the system. This measure proves advantageous because the rubber or rubber-like material forming the bearing element for the stylus holder must have a very high internal friction while its Shore hardness has to be small and, mostly, without the spring wire, shows a so-called tendency to creeping. The disadvantages of this arrangement are that, due to the use of the wire, resonances occur in the high and highest frequency range and that, due to the geometry of the bearing element and its connection to the casing, compression and tension forces appear which are effective at locations relatively remote from the center of motion and, because of the well-known structure of the mechanical system, are always somewhat unbalanced relative to the center of motion. Such lack of equilibrium causes an uneven frequency response of the pickup while, in addition, the relatively large mass of rubber or rubber-like material increases the total mass of the oscillating system and clips the frequency response in the upward direction.